Mel B -- Spice Up Your Life ... With Some TOPLESS Girl-on-Girl Action

GIRL POWER has just taken on a whole new meaning -- former Scary Spice Mel B stripped down to a bikini this weekend in Australia with a bunch of her lady friends ... and things got real frisky, real fast.

38-year-old Mel B and her gal pals sunned themselves on a yacht in Sydney Harbor Sunday. Mel's film producer husband Stephen Belafonte was there too, but her BFFs were the ones getting all the action.

"If you wanna be my lover ... you gotta get with my friends."